This research aims to overcome challenges in characterizing flood risk within the A Zone through a systematic approach. Specifically, this study sets out two main objectives: 1) provide a meaningful estimate of the range of expected annual flood risk in the A Zone, and 2) calculate the reduction in annual flood risk via elevation for homes in the A Zone. The lack and limited flood hazard data in the A Zone is addressed by developing a library of combinations of synthetic, regression-derived flood parameters that meet the mathematical definition of the A Zone.

Therefore, this paper addresses the challenges in characterizing flood risk in the A Zone. The approach resolves the Gumbel quantile function for four distinct flooding cases (i.e., location flooded at return periods exceeding 1.58-, 10-, 25-, and 50-year return period events), generating a library of synthetic flood parameters that meet the flood conditions in the A Zone. The method is used to assess the flood risk for hypothetical single-family homes with various features (i.e., one vs. two-plus stories, with vs. without basement) located in the A Zone in the United States. The study also explores the relative reduction in flood risk achieved with each additional first-floor elevation (FFE) increment (i.e., one to four feet) above the base flood elevation (BFE; i.e., freeboard). To validate and demonstrate the utility of the flood risk assessment generated here, real flood parameters are derived from available flood depth data at multiple return periods from various locations in California, Colorado, Michigan, New Hampshire, New Jersey, and Oregon. The results of the flood risk assessment using these real flood parameters are compared to those generated using the synthetic parameters.

The methodology to generate synthetic flood parameters consists of three steps. First, the Gumbel quantile function is resolved using the mathematical definition of A Zone flooding. Second, distinct flooding cases (i.e., location flooded at return periods exceeding 1.58, 10, 25, and 50 years) are defined to further explain flood risk using potentially available flood data. Finally, a library of synthetic flood parameters is generated through the definition of the range of each flood parameter and by resolving the ratio of flood parameters for each flood case. Using the library of synthetic characteristics and a new computational framework, average annual loss (AAL) for each case and the reduction with additional elevation above BFE are computed for a hypothetical single-family home with one- vs. two-plus stories, and with vs. without basement, located in the A Zone.

The validity of the results is confirmed by comparing AAL generated from synthetic parameters with that generated from real data in various locations in the United States.

The major findings are:

• The synthetic flood parameter approach generates multiple combinations of flood parameters for each flooding case, involving a wide range of 100-year flood depths.
• The flood depth-return period relationship for each flooding case, including all synthetic flood parameter combinations, provides vital information about flood depth by return period.
• The AAL for all flooding cases, taking into account four types of single-family homes (one-story and two-plus-story, without basement and with basement) in the A Zone ranges from 0.3 to 1.0 percent of the replacement cost value when considering the analyzed synthetic data.
• The relative flood risk is reduced by more than 90% by adding one foot of freeboard if the annual flood risk falls between the minimum and 25th percentiles with a 100-year flood depth of less than two feet. For areas falling between the 75th percentile and maximum annual flood risk, three feet of freeboard is needed to reduce flood risk by 50%.
• The descriptive statistical results for flood parameters, risk, and risk reduction with additional freeboard enhance the understanding of flood risk and the benefits of elevating above the BFE.
• For case study areas in California, Colorado, Michigan, New Hampshire, New Jersey, and Oregon, AAL values calculated from flood depth grids fall within the range of those computed from synthetic flood parameters.

The results provide an important first step for predicting and enhancing community understanding of flood risk even in the absence of flood depth data. The approach not only fills a critical gap by estimating the range of 100-year flood depth in A Zone but also provides insights into flood risk as a proportion of replacement cost value. The incorporation of the elevation strategy enhances awareness, emphasizing the importance of adopting mitigation measures in flood-prone areas. Furthermore, providing specific recommended elevation for each location contributes to enhanced individual- and community-level awareness. These features make the

approach widely applicable as populations continue to increase in areas in which the flood risk is unknown due to absent or outdated data.

To see the results in detail and read more about our peer-reviewed publication on this research click on the link below:

Theoretical boundaries of annual flood risk for single-family homes within the 100-year floodplain

Al Assi, A., Mostafiz, R.B., Friedland, C.J., and Rohli, R.V. (2024). Theoretical boundaries of annual flood risk for single-family homes within the 100-year floodplain. International Journal of Environmental Research, 18, Art. No. 29. doi: 10.1007/s41742-024-00577-7